Web Content sometimes not loading

I’ve created two custom apps via the App Studio. Very simple: both just include one tab which loads a standard URL to show the web content within Teams.

Both apps are personal tabs so they can get pinned on the left side.

For me they are working, also for some others in our tenant. But for some only a blank page loads.

One tab is pointing to https://outlook.office365.com/, the other one to a SharePoint aspx file.

Loading the sites in an ordinary web browser akways works for the others.

Any hints why it’s not working within teams sometimes?

Thanks! Marc